These days, the development of polymer electrolyte fuel cells, as fuel cells for automobiles, is beginning to progress rapidly. The polymer electrolyte fuel cell is a fuel cell in which hydrogen and oxygen are used and an organic substance film of a hydrogen-ion-selective permeability type (also the development of compositing with an inorganic substance is in progress) is used as the electrolyte. As the hydrogen of fuel, hydrogen gas obtained by the reforming of alcohols etc. is used as well as pure hydrogen.
However, in the existing fuel cell systems, the unit prices of components and members are high, and large decreases in the costs of the components and members are essential for the application to consumer products. In the application to automobile uses, not only cost reduction but also the compactification of a stack that forms the heart of the fuel cell is desired.
The polymer electrolyte fuel cell has a structure in which separators push both sides of a unit called a membrane electrode assembly (hereinafter occasionally referred to as an “MEA”) in which a polymer electrolyte film, an electrode, and a gas diffusion layer are integrated, and this structure is stacked in multiple layers to form a stack.
The properties required for the separator are to have electron conductivity, isolation properties between the oxygen gas and the hydrogen gas of both electrodes, low contact resistance with the MEA, good durability in the environment in the fuel cell, etc. Here, of the MEA, the gas diffusion layer (GDL) is generally made of carbon paper in which carbon fibers are integrated, and hence it is desired for the separator to have good contact-to-carbon electrical conductivity.
Stainless steel, titanium material, etc. as the material for the separator generally have low contact-to-carbon electrical conductivity in the state as they are, and hence many proposals has been made in order to enhance the contact-to-carbon electrical conductivity. The presence of a passive film with low electrical conductivity is an obstacle to enhance the contact-to-carbon electrical conductivity. Although this problem can be solved by sacrificing the durability, the interior of the fuel cell becomes a severe corrosion environment, and hence very high durability is required for the separator.
Thus, it is a reality that the development of a satisfying metal material for the separator is extremely difficult. Thus far, a carbon separator has been the mainstream, but when a metal separator is put to practical use, the fuel cell itself can be compactified and it can be ensured that cracking does not occur during the fuel cell production process; thus, the metallization of the separator is essential for mass production and spread.
In such a background, for example, Patent Literature 1 discloses a technology in which a special stainless steel in which a compound having electrical conductivity is precipitated in steel material is used from the viewpoints of thinness, weight reduction, etc. and thus the contact resistance of the stainless steel is allowed to be effectively reduced.
Also the use of titanium, which has good durability, for the separator is being investigated. Also in the case of titanium, the contact resistance with the MEA is high due to the presence of a passive film on the outermost surface of the titanium, like in stainless steel. Thus, for example, Patent Literature 2 discloses a technology in which a TiB-based precipitate is dispersed in titanium to reduce the contact resistance with the MEA.
Patent Literature 3 discloses a titanium alloy for a separator made of a titanium alloy in which Ta is contained at 0.5 to 15 mass % and the amounts of Fe and O are restricted as necessary, and in which the average nitrogen concentration of the area extending 0.5 μm in depth from the outermost surface is 6 atomic % or more and tantalum nitride and titanium nitride are present in the area.
Patent Literature 3 also discloses a method for producing a titanium alloy for a separator, in which heating is performed in the temperature range of 600 to 1000° C. for 3 seconds or more in a nitrogen atmosphere.
Patent Literatures 4, 5 and 6 disclose technologies in which an electrically conductive substance is pushed into an outer layer portion by the blasting method or the roll processing method in the production process of a metal separator made of titanium or stainless steel. In this technology, both to-carbon electrical conductivity and durability are achieved by a surface fine structure in which the electrically conductive substance is placed so as to penetrate through a passive film of the metal surface.
Patent Literature 7 discloses a method for producing a fuel cell separator in which an impurity containing titanium carbide or titanium nitride formed on a titanium surface is converted to an oxide by anodic oxidation treatment and then plating treatment is performed. The titanium carbide or the titanium nitride formed on the titanium surface is dissolved during the exposure to a corrosion environment and is re-precipitated as an oxide that inhibits the contact electrical conductivity, and reduces the contact electrical conductivity.
The method mentioned above suppresses the oxidation of the impurity during electricity generation (during use), and thus enhances the durability. However, an expensive plating film is indispensable in order to ensure electrical conductivity and durability.
Patent Literature 8 discloses a technology in which a titanium-based alloy obtained by alloying a group 3 element of the periodic table is used as the base material, BN powder is applied to the surface of the base material, and heating treatment is performed to form an oxide film to form a corrosion-resistant electrically conductive film.
This technology enhances the electrical conductivity by doping an impurity atom into the position of a titanium atom in the crystal lattice of the oxide film, which forms a passive film of the titanium alloy.
Patent Literatures 9 and 10 disclose technologies in which, when a fuel cell separator made of titanium is subjected to rolling processing, a carbon-containing rolling oil is used to perform rolling to form an altered layer containing titanium carbide on the outer layer and form thereon a carbon film with high film density, and electrical conductivity and durability are thus ensured.
In these technologies, although the electrical conductivity to carbon paper is enhanced, the durability is maintained by the carbon film and hence it is necessary to form a dense carbon film. Since contact resistance is high at a simple interface between carbon and titanium, titanium carbide, which enhances the electrical conductivity, is placed therebetween. However, if there is a defect in the carbon film, the corrosion of the altered layer (containing titanium carbide) and the base material cannot be prevented, and a corrosion product that inhibits the contact electrical conductivity may be produced.
Patent Literatures 11, 12, 13, 14 and 15 disclose titanium and fuel cell separators made of titanium that include, as the main structure, a carbon layer/a titanium carbide intermediate layer/a titanium base material, whose structures are similar to the structure described in Patent Literature 9. Although the production procedure of forming a carbon layer beforehand and then forming a titanium carbide intermediate layer is different from the production procedure described in Patent Literature 9, the mechanism of enhancing the durability by means of a carbon layer is similar.
Patent Literature 16 discloses a technology in which, for the purpose of mass production, graphite powder is applied and rolling is performed, and annealing is performed. This technology has achieved the function of the conventional carbon separator by providing a carbon layer and a titanium carbide intermediate layer on the base material titanium surface free from cracking. However, the titanium carbide intermediate layer does not have durability; hence, if there is a defect in the carbon layer, the corrosion of the titanium carbide intermediate layer and the base material cannot be prevented, and it is concerned that a surface structure could allow the production of a corrosion product that inhibits the contact electrical conductivity.
In this actual situation, Patent Literature 17 discloses a technology in which titanium carbide or titanium nitride, as an electrically conductive substance, is placed on a titanium surface, and these electrically conductive substances as well as the titanium are covered with a titanium oxide having passivation action. Although this technology ensures a contact electrical conductivity and also improves the durability, in order to further prolong the fuel cell lifetime, it is necessary to further enhance the environmental deterioration resistance of the titanium oxide film that covers the electrically conductive substance.
Thus, the present applicant has proposed, in Patent Literature 18, a titanium or a titanium alloy material for a fuel cell separator in which, while durability enhancement by subjecting a titanium oxide film to a passivation treatment in which immersion is performed in an aqueous solution containing an oxidizing agent such as nitric acid or chromic acid is taken as a basis, titanium compound particles containing carbon or nitrogen, which are minute electrically conductive objects, are dispersed in the oxide film of the surface of the titanium or the titanium alloy material, and thus the contact-to-carbon electrical conductivity is enhanced.
The present applicant has proposed, in Patent Literature 19, using a carbide, a nitride, a carbonitride, or a boride of tantalum, titanium, vanadium, zirconium, or chromium as minute electrically conductive objects and performing stabilization treatment after passivation treatment in aqueous solutions. The stabilization treatment uses an aqueous solution containing rice flour, wheat flour, potato starch, corn flour, soybean flour, a pickling corrosion inhibitor, or the like, which is a naturally derived substance or an artificially synthesized substance containing one or more of an amine-based compound, an aminocarboxylic acid-based compound, a phospholipid, a starch, calcium ions, and polyethylene glycol.
The internal environment of the polymer electrolyte fuel cell and the conditions of simulation evaluations thereof will now be described.
Patent Literatures 20, 21, 22, 23 and 24 discloses that a minute amount of fluorine is dissolved out and a hydrogen fluoride environment is produced when a fluorine-based polymer electrolyte is used for the electrolyte film. It is presumed that there is no dissolving-out of fluorine from the electrolyte film when a hydrocarbon polymer is used.
Patent Literature 24 discloses that the pH of a discharged liquid is made approximately 3 experimentally. In Patent Literature 10, a potentiostatic corrosion test in which an electric potential of 1 V is applied in a sulfuric acid aqueous solution at pH 4 and 50° C. is employed, and in Patent Literatures 11, 12, 13 and 14, a durability evaluation test in which an electric potential of 0.6 V is applied in a sulfuric acid aqueous solution at approximately pH 2 and 80° C. is employed.
Patent Literature 25 discloses an operating temperature being 80 to 100° C. In Patent Literatures 21 and 24, 80° C. is used as an evaluation condition. From the above, it is easily supposed that the evaluation conditions for simulating a polymer electrolyte fuel cell are (1) an aqueous solution at pH 2 to 4 in which fluorine is dissolved due to the polymer electrolyte of the electrolyte film, (2) a temperature of 50 to 100° C., and (3) a cell voltage change of 0 to 1 V (the voltage being 0 before electricity generation).
On the other hand, from the viewpoint of the environment resistance of titanium, it is known that titanium is dissolved in a hydrogen fluoride aqueous solution (hydrofluoric acid). Non-Patent Literature 1 discloses that the color change of titanium is promoted when fluorine is added at approximately 2 ppm or approximately 20 ppm to a sulfuric acid aqueous solution at pH 3.
Patent Literature 26 discloses a method in which a titanium alloy containing one or more elements of the platinum group-based elements (Pd, Pt, Ir, Ru, Rh, and Os), Au, and Ag is immersed in a non-oxidizing acid to form on the surface a layer containing them in a total amount of 40 to 100 atomic %. Patent Literature 27 discloses a titanium material for a separator in which a titanium alloy containing 0.005 to 0.15 mass % of one or more platinum group elements and 0.002 to 0.10 mass % of one or more rare-earth elements is pickled with a non-oxidizing acid to concentrate the one or more platinum group elements on the surface. Patent Literature 28 discloses a titanium material comprising a layer containing a titanium hydride in the titanium material surface.
The color change phenomenon described in Patent Literature 25 is a phenomenon in which interference colors occur as a result of the fact that titanium is dissolved and re-precipitated as an oxide on the surface and an oxide film has grown. Since the re-precipitated oxide is a substance that inhibits the contact electrical conductivity as described above, the environment in which fluorine is dissolved out in the fuel cell is a more severe environment to titanium; thus, it is necessary to further enhance the durability in order not to increase the contact resistance.